The objects and familiars that affect Stat Gains from Fights do not affect monster levels. For example, bringing a Blood-Faced Volleyball familiar will increase your stats per fight but will not make the fight more difficult.

There are some Special Monsters which are exceptions to these rules. Also, Monster Level modifiers are not applied to monsters that are created during combat with a different monster (for example, amphibians produced by CLEESH, or tomb rat kings).

Colloquially, the term "Monster Level" (in the context of an absolute value, as opposed to "+X to Monster Level") is often used to mean "the value from which the monster's stat gains are calculated". This value is actually the monster's attack. Some people use these terms interchangeably.

Bonus Monster Abilities

Since the May 14, 2014 revamp, running high amounts of +ML (Monster Level modifiers) gives the monsters new abilities, making them even more challenging. The exact details of these abilities are still being determined.

Beginning at +151 ML, monsters gain stagger immunity, and may land ordinary attacks even when heavily outmoxied (needs spading)

If your ML bonus is negative, such as from equipping a Drowsy Sword, your stat gains are only determined by the monster's attack divided by 4.

Strategy Implications

Based on this, here are some basic adventuring strategies:

Know the peak monster level of an area, and don't enter it until your Moxie is at least 10 greater than that level, guaranteeing that you can only be hit by monster critical hits.

Know the average monster level of an area, and go ahead and enter that area when your attack stat is equal to or greater than that level, so that you will hit at least 50% of the time, but you will have to heal during or after each battle.

In combination with either of the above, wear equipment that raises monster levels to set up the monsters you will encounter with higher stat point gains, and then use combat skills that lower monster level to make the monsters beatable again (although you will still have to deal damage equal to the raised hit point value).

Manipulating +ML is a cornerstone of most speed ascension strategies. The gist of it is: pump up +ML to the highest point you can, while still reliably winning battles.

History

Before November 30, 2005 a monster's hit points did not increase with its level. This allowed the LTSynergy strategy to be very effective before this "nerf."

Between the advent of NS13 and January 15, 2009, running +ML would counteract the effects of running +noncombats.

A hidden Combat Frequency modifying effect was added to ML increasers with NS13, leading to an anti-synergy between net combat decreasers and +ML. This anti-synergy had no effect on the base combat rate, and caused a flat 1% combats per +5ML applied. Monster Aggravation Devices were a special case, and did not affect noncombat frequency.

This mechanic was removed with the trivial announcement: "The much-maligned, super-secret hidden interaction between +Monster Level and +Noncombat Chance has been removed."

From NS13 until May 14, 2014 stat gains from non-special monsters were calculated from the monster's total attack (base attack +variance +ML) by dividing by 4. The 2014 revamp changed stat-gain to ((base attack + variance)/4) + (ML/3), and introduced the extra ML "penalties" (monster abilities).

The concept of increasing stat gains from combat, while also making +ML more difficult to survive, debuted with the Class Act II: A Class For Pigs challenge path. Both the stat gains and the penalties were reduced from the CA2 levels before going into the base game.